Optical add/drop device

ABSTRACT

Disclosed herein is an optical add/drop device including an optical demultiplexer for separating WDM signal light into n optical signals where n is an integer not less than 2, n first optical switches to which the n optical signals output from the optical demultiplexer are supplied, respectively, a second optical switch for adding provided on the input side of the first optical switches, a third optical switch for dropping provided on the output side of the first optical switches, n regenerators for wavelength converting the optical signals passed through the first optical switches, and an optical multiplexer for wavelength division multiplexing the wavelength-converted optical signals. A transmission distance between nodes can be increased by using this device.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical add/drop device.

[0003] 2. Description of the Related Art

[0004] With a recent increase in traffic, a large-capacity network hasbeen required. To meet this requirement, the construction of a WDM(wavelength division multiplexing) based optical network will benecessary in a future trunk network. An optical add/drop device (opticalADM: optical add/drop multiplexer) capable of adding and/or droppingoptical signals having different wavelengths by the wavelength betweenthis device and another network device such as a router and an ATM(asynchronous transfer mode) switch is used in an optical network. It isimportant to form a ring network by using this optical ADM and to forman optical network tolerant of failure.

[0005]FIG. 1 is a block diagram of a general optical add/drop device inthe prior art. An optical add/drop device 2 is arranged along an opticaltransmission line 4 for transmitting WDM signal light obtained bywavelength division multiplexing a plurality of optical signals havingdifferent wavelengths Al to An (n is an integer satisfying 1<n). Thisdevice 2 has an optical demultiplexer 6 for separating the WDM signallight supplied from the input-side optical transmission line 4 into theoptical signals having the wavelengths λ1 to λn, n 2×2 optical switches8 for selecting optical signals to be passed through this device 2 andoptical signals to be added/dropped, and an optical multiplexer 10 forwavelength division multiplexing optical signals to be output to theoutput-side optical transmission line 4.

[0006] k (k is an integer satisfying 1<k) regenerators 14 each having awavelength conversion function are provided for an interface betweenthis device 2 and another network device for an add signal, such as arouter 12, and k regenerators 18 each having a wavelength conversionfunction are provided for an interface between this device 2 and anothernetwork device for a drop signal, such as an ATM switch 16.

[0007] Examples of each optical switch include a waveguide switch usinga thermooptic effect and a mechanical switch having a motor. Examples ofa wavelength converter usable as each regenerator include an all-opticalwavelength converter using a semiconductor optical amplifier and awavelength converter using opto/electrical conversion andelectro/optical conversion. Examples of the optical multiplexer or theoptical demultiplexer include a device using an arrayed waveguidegrating (AWG) and a device using a dielectric multilayer film.

[0008] A device controller 20 is provided in the optical add/drop device2 to perform setting or the like for signal adding, dropping, or passingaccording to instructions from an operation system 22 for supervisingand controlling the whole of an optical network. In this manner, theoptical add/drop device 2 selects signal adding, dropping, or passing tothereby allow a flexible operation of the optical network.

[0009] In the device shown in FIG. 1, an optical signal from the router12 is wavelength converted by each regenerator 14 and accommodated intothe optical network at the wavelength assigned to an input port of theoptical multiplexer 10. That is, wavelengths are fixedly assigned in theoptical network. For more flexible and efficient operation of theoptical network, it is desirable that an optical signal can beaccommodated into the optical network at an arbitrary wavelength.

[0010]FIG. 2 is a block diagram of an improved optical add/drop devicein the prior art. In this device, k tunable wavelength converters 28 areused in place of the k regenerators 14 shown in FIG. 1. Further, a k×noptical switch 24 is provided between the tunable wavelength converters28 and the optical switches 8, and an n×k optical switch 26 is providedbetween the regenerators (wavelength converters) 18 and the opticalswitches 8. The optical switches 24 and 26 may be replaced by AWGs. Thenumber k represents the number of ports of an intraoffice interfacebetween this device and another network device such as a router 12 andan ATM switch 16, and the number n represents the number of WDMchannels. Accordingly, k≦n in general.

[0011] In the improved optical add/drop device mentioned above, anoptical signal from the router 12 can be wavelength converted to beadded to an arbitrary wavelength channel owing to the use of the tunablewavelength converters 28 and the optical switch 24. In case ofconverting the wavelength of an optical signal to be added into awavelength λn, the optical switch 24 is controlled to perform routing sothat the optical signal from the router 12 is passed through the opticalswitch 8 connected to the input port of the optical multiplexer 10corresponding to the wavelength λn.

[0012] In the prior art, the optical signals passing through the opticaladd/drop device are not subjected to wavelength conversion. Accordingly,a function of waveform shaping or the like associated with signalregeneration processing cannot be obtained, so that long-haultransmission is difficult to attain.

[0013] Further, in the prior art, a tunable wavelength converterincluding a tunable light source is necessary at an interface betweenthe optical add/drop device and another network device such as a routerand an ATM switch, so as to add a signal from the other network deviceto an arbitrary wavelength channel in the optical network.

[0014] Referring to FIGS. 3A and 3B, there are shown different tunablelight sources in the prior art. In the configuration shown in FIG. 3A, aplurality of laser diodes (LD1 to LDn) 30 are kept steadily driven, andCW light (DC light or unmodulated light) output from one of the laserdiodes 30 selected by an n×1 optical switch 32 is modulated by anoptical modulator (Mod) 34 to obtain an optical signal having a desiredwavelength. An optical signal from another network device is convertedinto an electrical signal by an opto/electrical (O/E) converter 36, andthis electrical signal is supplied as a modulating signal to the opticalmodulator 34. The optical switch 32 is controlled by a device controller38.

[0015] In this configuration, the time for switching wavelengths dependson the transit time of the optical switch 32. Further, there is apossibility of crosstalk in the optical switch 32 and an increase incost and power consumption.

[0016] In the configuration shown in FIG. 3B, a plurality of drivecircuits (DRV) 42 respectively associated with a plurality of laserdiodes 40 are turned on/off by a device controller 44 to select one ofthe laser diodes 40 and steadily drive it. CW light output from theselected laser diode 40 is modulated by an optical modulator 48 tothereby obtain an optical signal having a desired wavelength. An opticalsignal from another network device is converted into an electricalsignal by an opto/electrical converter 50, and this electrical signal issupplied as a modulating signal to the optical modulator 48. The plurallaser diodes 40 and the optical modulator 48 are optically connected byan AWG 46.

[0017] Also in this configuration, the LDs and the DRVs whose numbercorresponds to the number of wavelengths for each are required assimilar to the previous configuration shown in FIG. 3A, causing anincrease in cost.

[0018] Thus, a tunable light source is required in the prior art, so asto add a signal from another network device to an arbitrary wavelengthchannel in the optical network, resulting in an increase in cost of theoptical add/drop device.

SUMMARY OF THE INVENTION

[0019] It is therefore an object of the present invention to provide anoptical add/drop device which can increase a transmission distancebetween nodes to thereby allow long-haul transmission.

[0020] It is another object of the present invention to provide alow-cost optical add/drop device which can eliminate the need for atunable wavelength converter including a tunable light source at aninterface between this device and another network device. Other objectsof the present invention will become apparent from the followingdescription.

[0021] In accordance with a first aspect of the present invention, thereis provided an optical add/drop device comprising an opticaldemultiplexer for separating WDM signal light into n (n is an integersatisfying 1<n) optical signals having different wavelengths, said WDMsignal light being obtained by wavelength division multiplexing said noptical signals; n first optical switches each having first and secondinput ports and first and second output ports, said n optical signalsoutput from said optical demultiplexer being supplied to said firstinput ports of said n first optical switches, respectively; a secondoptical switch having k (k is a natural number) input ports and n outputports, an optical signal to be added being supplied to at least one ofsaid k input ports of said second optical switch, said n output ports ofsaid second optical switch being connected to said second input ports ofsaid n first optical switches, respectively; n regenerators connected tosaid first output ports of said n first optical switches, respectively;an optical multiplexer for wavelength division multiplexing opticalsignals output from said n regenerators; and a third optical switchhaving n input ports and k output ports, said n input ports of saidthird optical switch being connected to said second output ports of saidn first optical switches, respectively, an optical signal to be droppedbeing output from at least one of said k output ports of said thirdoptical switch.

[0022] As each regenerator, a wavelength converter or anopto/electro/optical converter may be used.

[0023] In accordance with a second aspect of the present invention,there is provided an optical add/drop device comprising an opticaldemultiplexer for separating WDM signal light into n (n is an integersatisfying 1<n) optical signals having different wavelengths, said WDMsignal light being obtained by wavelength division multiplexing said noptical signals; n optical switches each having first and second inputports and first and second output ports, said n optical signals outputfrom said optical demultiplexer being supplied to said first input portsof said n optical switches, respectively; a first electrical switchhaving k (k is a natural number) input ports, s (s is a natural number)input ports, and n output ports; k first opto/electrical convertersconnected to said k input ports of said first electrical switch,respectively; n first electro/optical converters for connecting said noutput ports of said first electrical switch and said second input portsof said n optical switches, respectively; an optical multiplexer forwavelength division multiplexing optical signals output from said firstoutput ports of said n optical switches; a second electrical switchhaving n input ports, k output ports, and s output ports; n secondopto/electrical converters for connecting said n input ports of saidsecond electrical switch and said second output ports of said n opticalswitches, respectively; k second electro/optical converters connected tosaid k output ports of said second electrical switch, respectively; ands electrical links for connecting said s input ports of said firstelectrical switch and said s output ports of said second electricalswitch, respectively.

[0024] In accordance with a third aspect of the present invention, thereis provided an optical add/drop device comprising an opticaldemultiplexer for separating WDM signal light into n (n is an integersatisfying 1<n) optical signals having different wavelengths, said WDMsignal light being obtained by wavelength division multiplexing said noptical signals; n optical switches each having first and second inputports and first and second output ports, said n optical signals outputfrom said optical demultiplexer being supplied to said first input portsof said n optical switches, respectively; a first electrical switchhaving k (k is a natural number) input ports, s (s is a natural number)input ports, and n output ports; (k+s) first opto/electrical convertersconnected to said (k+s) input ports of said first electrical switch,respectively; n first electro/optical converters for connecting said noutput ports of said first electrical switch and said second input portsof said n optical switches, respectively; an optical multiplexer forwavelength division multiplexing optical signals output from said firstoutput ports of said n optical switches; a second electrical switchhaving n input ports, k output ports, and s output ports; n secondopto/electrical converters for connecting said n input ports of saidsecond electrical switch and said second output ports of said n opticalswitches, respectively; (k+s) second electro/optical convertersconnected to said (k+s) output ports of said second electrical switch,respectively; and s optical links for connecting said firstopto/electrical converters respectively corresponding to said s inputports of said first electrical switch and said second electro/opticalconverters respectively corresponding to said s output ports of saidsecond electrical switch, respectively.

[0025] In accordance with a fourth aspect of the present invention,there is provided an optical add/drop device adapted to first and secondoptical paths. This device comprises first and second optical add/dropunits connected to said first and second optical paths, respectively; anadd switch for selectively adding an optical signal to any one of saidfirst and second optical add/drop units; and a drop switch forselectively dropping an optical signal from any one of said first andsecond optical add/drop units. Each of said first and second opticaladd/drop units comprises an optical demultiplexer for separating WDMsignal light into n (n is an integer satisfying 1<n) optical signalshaving different wavelengths, said WDM signal light being obtained bywavelength division multiplexing said n optical signals; n opticalswitches each having first and second input ports and first and secondoutput ports, said n optical signals output from said opticaldemultiplexer being supplied to said first input ports of said n opticalswitches, respectively; n regenerators connected to said first outputports of said n optical switches, respectively; and an opticalmultiplexer for wavelength division multiplexing optical signals outputfrom said n regenerators. The add switch is connected to said secondinput ports of said n optical switches, and the drop switch is connectedto said second output ports of said n optical switches.

[0026] In accordance with a fifth aspect of the present invention, thereis provided an optical add/drop device comprising an optical switch foradding and dropping optical signals having different wavelengths, and aregenerator provided on the output side of said optical switch forconverting an arbitrary wavelength into a specific wavelength, whereinthe waveform shaping of an optical signal passed through said opticalswitch without dropping and the wavelength conversion of an arbitrarywavelength of an optical signal added into a specific wavelength areperformed.

[0027] The above and other objects, features and advantages of thepresent invention and the manner of realizing them will become moreapparent, and the invention itself will best be understood from a studyof the following description and appended claims with reference to theattached drawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a block diagram of a general optical add/drop device inthe prior art;

[0029]FIG. 2 is a block diagram of an improved optical add/drop devicein the prior art;

[0030]FIGS. 3A and 3B are block diagrams of different tunable lightsources in the prior art;

[0031]FIG. 4 is a block diagram showing a first preferred embodiment ofthe optical add/drop device according to the present invention;

[0032]FIG. 5 is a block diagram showing a second preferred embodiment ofthe optical add/drop device according to the present invention;

[0033]FIG. 6 is a block diagram showing a third preferred embodiment ofthe optical add/drop device according to the present invention;

[0034]FIG. 7 is a block diagram showing a fourth preferred embodiment ofthe optical add/drop device according to the present invention;

[0035]FIG. 8 is a block diagram showing an optical network system towhich the present invention is applicable;

[0036]FIG. 9 is a block diagram showing a fifth preferred embodiment ofthe optical add/drop device according to the present invention;

[0037]FIG. 10 is a block diagram showing a specific configuration of anelectrical switch for dropping shown in FIG. 9;

[0038]FIG. 11 is a block diagram showing a sixth preferred embodiment ofthe optical add/drop device according to the present invention;

[0039]FIGS. 12A and 12B are schematic diagrams for illustrating anexample of the operation of a 2F-UPSR (unidirectional path switched ringusing two fibers) to which the present invention is applicable;

[0040]FIG. 13 is a block diagram showing a seventh preferred embodimentof the optical add/drop device according to the present invention;

[0041]FIGS. 14A and 14B are block diagrams showing preferred embodimentsof an add switch and a drop switch shown in FIG. 13, respectively;

[0042]FIGS. 15A and 15B are block diagrams showing other preferredembodiments of the add switch and the drop switch shown in FIG. 13,respectively;

[0043]FIGS. 16A and 16B are schematic diagrams for illustrating anexample of the operation of a 4F-BPSR (bidirectional path switched ringusing four fibers) to which the present invention is applicable;

[0044]FIGS. 17A and 17B are schematic diagrams for illustrating anotherexample of the operation of the 4F-BPSR;

[0045]FIG. 18 is a block diagram showing an eighth preferred embodimentof the optical add/drop device according to the present invention;

[0046]FIGS. 19A and 19B are block diagrams showing first preferredembodiments of an add switch and a drop switch shown in FIG. 18,respectively;

[0047]FIGS. 20A and 20B are block diagrams showing second preferredembodiments of an add switch and a drop switch shown in FIG. 18,respectively;

[0048]FIGS. 21A and 21B are block diagrams showing third preferredembodiments of an add switch and a drop switch shown in FIG. 18,respectively; and

[0049]FIGS. 22A and 22B are block diagrams showing fourth preferredembodiments of an add switch and a drop switch shown in FIG. 18,respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] Some preferred embodiments of the present invention will now bedescribed in detail.

[0051]FIG. 4 is a block diagram showing a first preferred embodiment ofthe optical add/drop device according to the present invention. In thispreferred embodiment, a so-called DOADM (dynamic optical add/dropmultiplexer) is used as the optical add/drop device. The opticaladd/drop device is arranged along an optical transmission line(generally, an optical fiber transmission line) 4 for transmitting WDMsignal light obtained by wavelength division multiplexing a plurality ofoptical signals having different wavelengths λ1 to λn (n is an integersatisfying 1<n). The optical add/drop device includes an opticaldemultiplexer 6 for separating the WDM signal light supplied from theinput-side optical transmission line 4 into the optical signals of thewavelengths λ1 to λn, n optical switches 8 for selecting optical signalsto be passed through this optical add/drop device and optical signals tobe added/dropped, an optical multiplexer 10 for wavelength divisionmultiplexing optical signals to be output to the output-side opticaltransmission line 4, an optical switch 24 for adding an optical signal,and an optical switch 26 for dropping an optical signal.

[0052] Each optical switch 8 has two input ports 8A and 8B and twooutput ports 8C and 8D. The optical signals of the wavelengths λ1 to λnoutput from the optical demultiplexer 6 are supplied to the input ports8A of the n optical switches 8, respectively. Each optical switch 8 maybe provided by a 2×2 optical switch for switching between a bar stateand a cross state between the input ports 8A and 8B and the output ports8C and 8D.

[0053] The optical switch 24 has k (k is a natural number) input portsand n output ports. At least one of the k input ports of the opticalswitch 24 is supplied with an optical signal from another network devicefor an add signal, such as a router 12. The n output ports of theoptical switch 24 are connected to the input ports 8B of the n opticalswitches 8, respectively.

[0054] n regenerators 52 are connected to the output ports 8C of the noptical switches 8, respectively. The outputs of the n regenerators 52are connected to the n input ports of the optical multiplexer 10.

[0055] The optical switch 26 has n input ports and k output ports. The ninput ports of the optical switch 26 are connected to the output ports8D of the n optical switches 8, respectively, and an optical signal tobe dropped is output from at least one of the k output ports of theoptical switch 26. In this preferred embodiment, k regenerators 18 aswavelength converters are connected to the k output ports of the opticalswitch 26, respectively. The wavelength of the optical signal outputfrom at least one of the k output ports of the optical switch 26 isconverted into a desired wavelength by the corresponding regenerator 18,and the wavelength-converted optical signal is next supplied to anothernetwork device for a drop signal, such as an ATM switch 16.

[0056] The optical switches 24 and 26 may be provided by a k×n opticalswitch and an n×k optical switch, respectively.

[0057] The case of adding a desired optical signal having an arbitrarywavelength to the WDM signal light will now be described. The opticalsignal from the router 12, for example, is routed to the output port ofthe optical switch 24 connected to the optical switch 8 corresponding tothe desired wavelength, by setting the optical switch 24. This opticalswitch 8 is preliminarily controlled to have the cross state, so thatthe added optical signal is supplied to the corresponding regenerator 52to convert the wavelength of the added optical signal into a desiredwavelength. Then, the added optical signal is output through the opticalmultiplexer 10 to the output-side optical transmission line 4.

[0058] The case of dropping an optical signal having a desiredwavelength from the WDM signal light will now be described. The WDMsignal light supplied from the input-side optical transmission line 4 isseparated into a plurality of optical signals having differentwavelengths by the optical demultiplexer 6. The optical switch 8corresponding to the wavelength of the optical signal to be dropped ispreliminarily controlled to have the cross state. The optical signaldropped is routed to the desired output port of the optical switch 26 bysetting the optical switch 26, and the wavelength of the dropped opticalsignal is converted into the wavelength of an intraoffice interfacesignal, for example, by the corresponding regenerator 18. Thereafter,the wavelength-converted optical signal is supplied to another networkdevice such as an ATM switch 16. As the wavelength of the intraofficeinterface signal, a wavelength in a 1.3 μm band, for example, may beused. As the wavelength of each channel of the WDM signal light, awavelength in a 1.55 μm band, for example, may be used.

[0059] On the other hand, the optical signals passing through thisoptical add/drop device are subjected to wavelength conversion by theregenerators 52 by controlling the optical switches 8 so that they havethe bar state. The wavelength-converted optical signals from theregenerators 52 are output through the optical multiplexer 10 to theoutput-side optical transmission line 4.

[0060] According to this preferred embodiment, the regenerators 52corresponding to the wavelengths of the optical signals to be passed areprovided between the optical switches 8 and the optical multiplexer 10.Accordingly, all of the optical signals passing through this opticaladd/drop device and the optical signal added can be easily subjected tosignal processing such as waveform shaping. As a result, a transmissiondistance between optical add/drop devices can be greatly increased.

[0061] Further, the outputs of the regenerators 52 are preliminarilyassigned to the wavelengths of the WDM signal light. Accordingly, anoptical signal from another network device such as a router and an ATMswitch can be easily added to an arbitrary wavelength channel of the WDMsignal light without the need for use of wavelength converters eachincluding a tunable light source complicated in configuration as in theprior art.

[0062] Particularly in this preferred embodiment, the regenerators 18are connected to the output ports of the optical switch 26 for droppingan optical signal. Accordingly, the wavelength of the optical signaldropped can be easily made to coincide with the wavelength of anintraoffice interface signal.

[0063]FIG. 5 is a block diagram showing a second preferred embodiment ofthe optical add/drop device (DOADM) according to the present invention.In contrast to the configuration shown in FIG. 4, the second preferredembodiment is characterized by n regenerators (wavelength converters) 54provided on the input side of the optical switch 26 in place of theregenerators 18 provided on the output side of the optical switch 26.The n regenerators 54 are provided between the n input ports of theoptical switch 26 and the n optical switches 8, respectively.

[0064] The add operation and the through operation of the secondpreferred embodiment shown in FIG. 5 are the same as those of the firstpreferred embodiment shown in FIG. 4, so the drop operation will bedescribed herein. A plurality of optical signals separated by theoptical demultiplexer 6 are supplied to the n optical switches 8,respectively. The optical switch 8 corresponding to the wavelength of anoptical signal to be dropped is controlled to have the cross state. Thewavelength of the dropped optical signal is converted into thewavelength of an intraoffice interface signal, for example, by thecorresponding regenerator 54 before this optical signal is supplied tothe optical switch 26. Then, this optical signal is routed to anarbitrary port by the optical switch 26, and is next supplied to anothernetwork device such as an ATM switch 16.

[0065] Accordingly, this preferred embodiment can exhibit the effectobtained by the preferred embodiment shown in FIG. 4 and can furtherincrease the transmission distance between optical add/drop devicesbecause the regenerators 54 are provided on the input side of theoptical switch 26.

[0066]FIG. 6 is a block diagram showing a third preferred embodiment ofthe optical add/drop device (DOADM) according to the present invention.In contrast to the configuration shown in FIG. 4, the third preferredembodiment is characterized by n regenerators (wavelength converters) 56provided on the input side of the n optical switches 8 in place of theregenerators 18 provided on the output side of the optical switch 26.The n regenerators 56 are provided between the n output ports of theoptical demultiplexer 6 and the n optical switches 8, respectively.

[0067] The add operation and the through operation of the thirdpreferred embodiment shown in FIG. 6 are the same as those of the firstpreferred embodiment shown in FIG. 4, so the drop operation will bedescribed herein. The wavelengths of a plurality of optical signalsseparated by the optical demultiplexer 6 are converted into thewavelength of an intraoffice interface signal, for example, by theregenerators 56, and the wavelength-converted optical signals are nextsupplied to the optical switches 8. The optical switch 8 correspondingto the wavelength of an optical signal to be dropped is controlled tohave the cross state. Then, the dropped optical signal is routed to anarbitrary port by the optical switch 26, and is next supplied to anothernetwork device such as an ATM switch 16.

[0068] Accordingly, this preferred embodiment can exhibit the effectobtained by the preferred embodiment shown in FIG. 4 and can furtherincrease the transmission distance between optical add/drop devicesbecause the regenerators 56 are provided on the input side of theoptical switches 8.

[0069]FIG. 7 is a block diagram showing a fourth preferred embodiment ofthe optical add/drop device (DOADM) according to the present invention.In this preferred embodiment, optical switches 24′ and 26′ are used inplace of the optical switches 24 and 26 shown in FIG. 4, respectively.An m×n optical switch is used as the optical switch 24′, and an n×moptical switch is used as the optical switch 26′, where m is an integersatisfying m=k+r (r is a natural number). Accordingly, the opticalswitch 24′ has additional r input ports as compared with the opticalswitch 24, and the optical switch 26′ has additional r output ports ascompared with the optical switch 26. The remaining r input ports of theoptical switch 24′ and the remaining r output ports of the opticalswitch 26′ are connected by r links 58, respectively. Each link 58 maybe provided by an optical fiber link. The number r of links 58 may beequal to or less than the number n of channels of the WDM signal light.

[0070] According to the preferred embodiment shown in FIG. 7, wavelengthconversion can be applied to the optical signals passing through theoptical add/drop device, thereby allowing the construction or operationof a flexible optical network. This will now be described morespecifically.

[0071]FIG. 8 shows an optical network system to which the presentinvention is applicable. Reference symbols A to F denote nodes, and theoptical add/drop device according to the present invention is applicableto each node. These nodes are connected by an optical transmission line.

[0072] It is assumed that optical signals are transmitted between thenode B and the node C by using all of the wavelengths λ2 to λn. When anoptical signal having the wavelength λ2 is transmitted from the node Ato the node B, this optical signal cannot be transmitted at thewavelength λ2 from the node B to the node C, and it is thereforenecessary to perform the wavelength conversion from the wavelength λ2 ofthe optical signal to the remaining wavelength λ1. That is, wavelengthconversion must be applied to the optical signal passing through thenode B.

[0073] According to the preferred embodiment shown in FIG. 7, the aboverequirement can be easily met. The WDM signal light supplied from theinput-side optical transmission line 4 is separated into a plurality ofoptical signals by the optical demultiplexer 6, and these opticalsignals having different wavelengths are supplied to the opticalswitches 8, respectively. The optical switch 8 corresponding to thewavelength to be converted is preliminarily controlled to have the crossstate, so that this optical signal is supplied through the opticalswitch 26′ for dropping to any one of the r links 58, and is next inputinto the optical switch 24′ for adding. In the optical switch 24′, theinput optical signal is routed to the port connected to the opticalswitch 8 corresponding to a desired wavelength. This optical switch 8 ispreliminarily controlled to have the cross state, so that the opticalsignal added is passed through the optical switch 8 and next subjectedto wavelength conversion by the corresponding regenerator 52. Finally,this wavelength-converted optical signal is output at the desiredwavelength to the output-side optical transmission line 4.

[0074] According to the preferred embodiment shown in FIG. 7, it ispossible to prevent a reduction in utilization efficiency of a networkdue to no wavelength conversion in an optical add/drop device.

[0075]FIG. 9 is a block diagram showing a fifth preferred embodiment ofthe optical add/drop device (DOADM) according to the present invention.As similar to the previous preferred embodiments, the fifth preferredembodiment employs an optical demultiplexer 6, n optical switches 8, andan optical multiplexer 10. Further, these elements 6, 8, and 10 areconnected in a similar manner. In this preferred embodiment, electricalswitches 60 and 66 are used to provide processing of the optical signalspassing through the optical add/drop device by electrical path as aunit.

[0076] The electrical switch 60 has q input ports and n output ports,where q is an integer satisfying q=k+s (s is a natural number).Accordingly, a q×n electrical switch may be used as the electricalswitch 60.

[0077] k opto/electrical (O/E) converters 62 are connected to the kinput ports of the electrical switch 60, respectively. Eachopto/electrical converter 62 is provided for an intraoffice interfacebetween this optical add/drop device and another network device such asa router 12, for example.

[0078] n electro/optical (E/o) converters 64 are connected to the noutput ports of the electrical switch 60, respectively. The nelectro/optical converters 64 are connected to the input ports 8B of then optical switches 8, respectively.

[0079] The electrical switch 66 has n input ports and q output ports.Accordingly, an n×q electrical switch may be used as the electricalswitch 66. n opto/electrical converters 70 are connected to the n inputports of the electrical switch 66, respectively. The n opto/electricalconverters 70 are connected to the output ports 8D of the n opticalswitches 8, respectively. k electro/optical converters 68 are connectedto the k output ports of the electrical switch 66, respectively. Eachelectro/optical converter 68 is connected to another network device suchas an ATM switch 16.

[0080] The remaining s output ports of the electrical switch 66 and theremaining s input ports of the electrical switch 60 are connected by slinks 69, respectively. Each s link 69 is provided by an electricallink.

[0081] Referring to FIG. 10, there is shown a specific configuration ofthe electrical switch 66 shown in FIG. 9. The electrical switch 66includes a frame synchronizing circuit 72 connected to the nopto/electrical converters 70, n frame demultiplexing circuits 74connected to the frame synchronizing circuit 72, q frame multiplexingcircuits 78, and an electrical matrix switch 76 connected between theframe synchronizing circuits 74 and the frame multiplexing circuits 78.In the case that the number of frames to be multiplexed is 4, a 4n×4qswitch may be used as the electrical matrix switch 76. The s framemultiplexing circuits 78 of the q frame multiplexing circuits 78 areconnected through the s links 69 to the electrical switch 60 for adding,and the remaining k frame multiplexing circuits 78 are connected throughthe k electro/optical converters 68 to the other network device 16 (seeFIG. 9).

[0082] According to this preferred embodiment, for example, of 10 Gb/s(Oc-192 in SONET) assigned to one wavelength, 7.5 Gb/s (Oc-48 of threechannels) may be passed through the optical add/drop device, and 2.5Gb/s (Oc-48 of one channel) may be dropped from the optical add/dropdevice. This will now be described more specifically.

[0083] In the optical add/drop device shown in FIG. 9, the WDM signallight supplied from the input-side optical transmission line 4 to theoptical demultiplexer 6 is separated into a plurality of optical signalshaving wavelengths λ1 to λn, and these n optical signals are next inputinto the n optical switches 8, respectively. The optical switch 8corresponding to the wavelength of an optical signal required to beprocessed by electrical path as a unit is preliminarily controlled tohave the cross state. Accordingly, this optical signal is dropped inthis optical switch 8, and is next converted into an electrical signalby the corresponding opto/electrical converter 70. This electricalsignal is next input into the electrical switch 66.

[0084] An Oc-192 signal (10 Gb/s) as the electrical signal is subjectedto frame synchronization by the frame synchronizing circuit 72 shown inFIG. 10, and next separated into Oc-48 signals (2.5 Gb/s for each) offour channels by the corresponding frame demultiplexing circuit 74. Inthe electrical matrix switch 76, these Oc-48 signals are routed todesired paths by Oc-48 signal as a unit. These routed signals are nextmultiplexed by the corresponding frame multiplexing circuit 78 to obtainan Oc-192 signal again, which is in turn converted into an opticalsignal by the corresponding electro/optical converter 68. The signalhaving 7.5 Gb/s (Oc-48 of three channels) to be passed through theoptical add/drop device is routed to the port connected to theelectrical switch 60 by the corresponding link 69, and the drop signalhaving 2.5 Gb/s (Oc-48) is routed to the intraoffice interface. Thesignal to be passed is next routed by the electrical switch 60 to returnto the original optical switch 8. Since this optical switch 8 iscontrolled to have the cross state as mentioned above, the returnedsignal is added at the original wavelength to the WDM signal light, andthen output to the output-side optical transmission line 4.

[0085] The advantage of this operation over the prior art will now bedescribed with reference to FIG. 8. In the case that a signal of 10 Gb/sis transmitted from the node D to the node E, for example, there is acase that a signal of 2.5 Gb/s is desired to be dropped at the node Eand the remaining signal of 7.5 Gb/s is desired to be dropped at thenode F. In the prior art, such an operation requires the use of twowavelengths (λ3 and λ4 in FIG. 8) between the node D and the node E. Tothe contrary, the preferred embodiment shown in FIGS. 9 and 10 has anadvantage that the use of one wavelength is sufficient for thetransmission between the node D and the node E because the processing byelectrical path as a unit is allowed. Accordingly, it is possible toprevent a reduction in utilization efficiency of a network due to theprocessing by wavelength as a unit within an optical network.

[0086]FIG. 11 is a block diagram showing a sixth preferred embodiment ofthe optical add/drop device (DOADM) according to the present invention.This preferred embodiment will now be described in contrast to thepreferred embodiment shown in FIG. 9. The k opto/electrical converters62 provided on the input side of the electrical switch 60 for adding aremodified to q (q=k+s) opto/electrical converters 62′. The kelectro/optical converters 68 provided on the output side of theelectrical switch 66 for dropping are modified to q electro/opticalconverters 681. Further, the s links 69 for electrical signals aremodified to s links 69′ provided by optical links.

[0087] According to this preferred embodiment, the effect obtained bythe preferred embodiment shown in FIGS. 9 and 10 can be obtained.Additionally, even when the electrical switch 60 for adding and theelectrical switch 66 for dropping are placed apart from each other,these switches can be easily connected by the optical links.

[0088]FIGS. 12A and 12B show an example of the operation of a 2F-UPSR(unidirectional path switched ring using two fibers) to which thepresent invention is applicable. Nodes A to D each including the opticaladd/drop device according to the present invention are connected by amain fiber 4A and a standby fiber 4B so as to form a ring. The mainfiber 4A transmits WDM signal light in a clockwise direction, and thestandby fiber 4B transmits WDM signal light in a counterclockwisedirection.

[0089] In a normal operation shown in FIG. 12A, the main fiber 4A isused to form a main traffic leading from the node A through the node Bto the node C. Further, the standby fiber 4B is used to form alow-priority traffic leading from the node A through the node D to thenode C. In case of failure, the low-priority traffic may be cut for themain traffic.

[0090] When a failure such as a fiber break occurs between the node Band the node C, for example, as shown in FIG. 12B, an operation systemmanaging the whole of a network detects this failure, and switches fromthe main fiber 4A to the standby fiber 4B at the transmitting node A. Atthe same time, the operation system switches from the main fiber 4A tothe standby fiber 4B at the receiving node C. In this case, thelow-priority traffic is cut.

[0091]FIG. 13 is a block diagram showing a seventh preferred embodimentof the optical add/drop device (DOADM) according to the presentinvention. This preferred embodiment is applicable to the 2F-UPSR shownin FIGS. 12A and 12B. This optical add/drop device includes a WDM ringsection (corresponding to the first optical add/drop unit) 80 of a mainsystem (Work) inserted in the main fiber 4A, a WDM ring section(corresponding to the second optical add/drop unit) 82 of a standbysystem (Protect) inserted in the standby fiber 4B, and an add switch 84and a drop switch 86 both connected between the WDM ring sections 80 and82.

[0092] Each of the WDM ring sections 80 and 82 includes the opticaldemultiplexer 6, the n optical switches 8, the n wavelength converters52, and the optical multiplexer 10 shown in FIG. 4, for example. The addswitch 84 is connected to another network device such as a router 12 andan ATM switch 16 for outputting an optical signal to be added, and thedrop switch 86 is connected to another network device such as a router12 and an ATM switch 16 for receiving an optical signal to be dropped.

[0093] The add switch 84 is connected to the second input ports 8B ofthe n optical switches 8 in each of the WDM ring sections 80 and 82, andthe drop switch 86 is connected to the second output ports 8D of the noptical switches 8 in each of the WDM ring sections 80 and 82.

[0094]FIG. 14A is a block diagram showing a preferred embodiment of theadd switch 84 shown in FIG. 13, and FIG. 14B is a block diagram showinga preferred embodiment of the drop switch 86 shown in FIG. 13. The addswitch 84 includes two k×n optical switches 88 and 90, and n 1×2 opticalswitches 92 and n 2×1 optical switches 94 for switching the outputs fromthe optical switches 88 and 90. Further, the drop switch 86 includes twon×k optical switches 96 and 98, and n 2×1 optical switches 100 and n 1×2optical switches 102 for switching the inputs to the optical switches 96and 98.

[0095] The operation of the add switch 84 will now be described withreference to FIG. 14A. In the main system, an optical signal fromanother network device selected by the k×n optical switch 88 is normallyswitched to the main fiber 80 by the corresponding 1×2 optical switch92, whereas in case of failure, the optical signal is switched to thestandby fiber 82 by the corresponding 1×2 optical switch 92. In thestandby system, an optical signal from another network device selectedby the k×n optical switch 90 is normally output through thecorresponding 2×1 optical switch 94 to the standby fiber 82, whereas incase of failure, an optical signal from another network device selectedby the k×n optical switch 88 is output through the corresponding 1×2optical switch 92 and the corresponding 2×1 optical switch 94 to thestandby fiber 82.

[0096] The operation of the drop switch 86 will now be described withreference to FIG. 14B. In the main system, each 2×1 optical switch 100normally selects an output from the main fiber 80, whereas in case offailure, each 2×1 optical switch 100 selects an output from the standbyfiber 82. Then, the selected output is supplied to another networkdevice selected by the n×k optical switch 96. In the standby system,each 1×2 optical switch 102 normally supplies an optical signal from thestandby fiber 82 to another network device selected by the n×k opticalswitch 98, whereas in case of failure, each 1×2 optical switch 102supplies this optical signal to another network device selected by then×k optical switch 96.

[0097] In any case, the k×n optical switches 88 and 90 control so thatthe wavelength conversion to a desired wavelength in the ring isperformed, and the n×k optical switches 96 and 98 control so that anoptical signal is output to a desired port.

[0098] Generally, the number k of ports for adding/dropping is smallerthan the number n of wavelengths of the WDM signal light, so that (n−k)low-priority signals can be transmitted even in case of failure.

[0099] Referring to FIGS. 15A and 15B, there are shown other preferredembodiments of the add switch 84 and the drop switch 86 shown in FIG.13, respectively. The preferred embodiment shown in FIG. 15A employs n1×2 optical couplers 104 in place of the n 1×2 optical switches 92 shownin FIG. 14A. The preferred embodiment shown in FIG. 15B employs n 1×2optical couplers 106 in place of the n 1×2 optical switches 102 shown inFIG. 14B. Also in these preferred embodiments, the switching in case offailure can be easily performed in the optical add/drop device in asimilar manner.

[0100] Referring to FIGS. 16A and 16B and FIGS. 17A and 17B, there areshown examples of the operation of a 4F-BPSR (bidirectional pathswitched ring using four fibers) to which the present invention isapplicable. Nodes A to D each including the optical add/drop deviceaccording to the present invention are connected by two main fibers4A(#1) and 4A(#2) and two standby fibers 4B(#1) and 4B(#2) so as to forma ring. The main fiber 4A(#1) and the standby fiber 4B(#1) transmit WDMsignal light in a counterclockwise direction, and the main fiber 4A(#2)and the standby fiber 4B(#2) transmit WDM signal light in a clockwisedirection.

[0101] In a normal operation shown in FIGS. 16A and 17A, the main fiber4A(#1) is used to form a main traffic leading from the node C throughthe node B to the node A, and the main fiber 4A(#2) is also used to formanother main traffic leading from the node A through the node B to thenode C. Further, in the normal operation, the standby fiber 4B(#1) isused to form a low-priority traffic leading from the node A through thenode D to the node C, and the standby fiber 4B(#2) is also used to formanother low-priority traffic leading from the node C through the node Dto the node A. In case of failure, a part or the whole of thelow-priority traffic may be cut for the main traffic.

[0102] When a failure such that all the optical transmission lines arebroken between the node B and the node C occurs, for example, as shownin FIG. 16B, an operation system managing the whole of a network detectsthis failure, and switches from the main fiber 4A(#1) to the standbyfiber 4B(#2) whose transmission direction is opposite to that of themain fiber 4A(#1). Further, the operation system switches from the mainfiber 4A(#2) to the standby fiber 4B(#1) whose transmission direction isopposite to that of the main fiber 4A((#2).

[0103] In the case that a failure occurs in only the main fiber 4A(#1)between the node B and the node C, for example, as shown in FIG. 17B,the main fiber 4A(#1) is switched to the standby fiber 4B(#1) whosetransmission direction is the same as that of the main fiber 4A(#1).

[0104] Thus, it is required to design such a configuration that the mainfiber 4A(#1) can be switched to the standby fiber 4B(#1) or 4B(#2) andthat the main fiber 4A(32) can also be switched to the standby fiber4B(#1) or 4B(#2).

[0105]FIG. 18 is a block diagram showing an eighth preferred embodimentof the optical add/drop device (DOADM) according to the presentinvention. This preferred embodiment is applicable to the 4F-BPSR shownin FIGS. 16A and 16B and FIGS. 17A and 17B. This optical add/drop deviceincludes two WDM ring sections 80(#1) and 80(#2) of a main systeminserted in the main fibers 4A(#1) and 4A(#2), respectively, two WDMring sections 82(#1) and 82(#2) of a standby system inserted in thestandby fibers 4B(#1) and 4B(#2), respectively, and an add switch 84 anda drop switch 86 both connected between the WDM ring sections 80(#1) and80(#2) and the WDM ring sections 82(#1) and 82(#2).

[0106] Each of the WDM ring sections 80(#1), 80(#2), 82(#1), and 82(#2)includes the optical demultiplexer 6, the n optical switches 8, the nwavelength converters 52, and the optical multiplexer 10 shown in FIG.4, for example. The add switch 84 is connected to another network devicesuch as a router 12 and an ATM switch 16 for outputting an opticalsignal to be added, and via wavelength converter 18, the drop switch 86is connected to another network device which receives an optical signalto be dropped to a router 12 and an ATM switch 16.

[0107] The add switch 84 is connected to the second input ports 8B ofthe n optical switches 8 in each of the WDM ring sections 80(#1),80(#2), 82(#1), and 82(#2), and the drop switch 86 is connected to thesecond output ports 8D of the n optical switches 8 in each of the WDMring sections 80(#1), 80(#2), 82(#1), and 82(#2).

[0108]FIG. 19A is a block diagram showing a first preferred embodimentof the add switch 84 shown in FIG. 18, and FIG. 19B is a block diagramshowing a firs preferred embodiment of the drop switch 86 shown in FIG.18. As shown in FIG. 19A, the add switch 84 includes two k×n opticalswitches 88(#1) and 88(#2) for a main system, and two k×n opticalswitches 90(#1) and 90(#2) for a standby system. The add switch 84further includes k 1×3 optical switches 108(#1), k 1×3 optical switches108(#2), k 3×1 optical switches 110(#1), and k 3×1 optical switches110(#2), so as to switch each input of the optical switches 88(#1) and88(#2) to each input of the optical switches 90(#1) and 90(#2).

[0109] In the add switch 84 shown in FIG. 19A, the optical signal to beadded from another network device is switched by the corresponding 1×3optical switch 108(#1) or 108(#2). Then, this optical signal is normallyinput into the main fiber 80(#1) or 80(#2), whereas in case of failure,this optical signal is input into the standby fiber 82(#1) or 82(#2). Tothis end, the optical switches 88(#1) and 88(#2), the optical switches90(#1) and 90(#2), the optical switches 108(#1) and 108(#2), and theoptical switches 110(#1) and 110(#2) are interconnected.

[0110] As shown in FIG. 19B, the drop switch 86 includes two n×k opticalswitches 96(#1) and 96(#2) for a main system, and two n×k opticalswitches 98(#1) and 98(#2) for a standby system. The drop switch 86further includes k 1×3 optical switches 114(#1), k 1×3 optical switches114(#2), k 3×1 optical switches 116(#1), and k 3×1 optical switches116(#2), so as to switch each output of the optical switches 96(#1) and96(#2) to each output of the optical switches 98(#1) and 98(#2).

[0111] In the drop switch 86 shown in FIG. 19B, the switching isperformed so that the output from the main fiber 80(#1) or 80(#2) isnormally dropped to another network device, whereas in case of failure,the output from the standby fiber 82(#1) or 82(#2) is dropped to anothernetwork device. To this end, the optical switches 96(#1) and 96(#2), theoptical switches 98(#1) and 98(#2), the optical switches 114(#1) and114(#2), and the optical switches 116(#1) and 116(#2) areinterconnected.

[0112] In any case, the k×n optical switches 88(#1), 88(#2), 90(#1), and90(#2) control so that the wavelength conversion to a desired wavelengthin the ring is performed, and the n×k optical switches 96(#1), 96(#2),98(#1), and 98(#2) control so that an optical signal is output to adesired port.

[0113] Generally, the number k of ports for adding/dropping is smallerthan the number n of wavelengths of the WDM signal light, so that (n−k)low-priority signals can be transmitted even in case of failure.

[0114]FIG. 20A is a block diagram showing a second preferred embodimentof the add switch 84 shown in FIG. 18, and FIG. 20B is a block diagramshowing a second preferred embodiment of the drop switch 86 shown inFIG. 18. In the preferred embodiment shown in FIG. 20A, 1×3 opticalcouplers 118(#1) and 118(#2) are used in place of the 1×3 opticalswitches 108(#1) and 108(#2) shown in FIG. 19A, respectively. In thepreferred embodiment shown in FIG. 20B, 1×3 optical couplers 120(#1) and120(#2) are used in place of the 1×3 optical switches 114(#1) and114(#2) shown in FIG. 19B, respectively. Also in these preferredembodiments, the switching in case of failure can be easily performed inthe optical add/drop device in a similar manner.

[0115]FIG. 21A is a block diagram showing a third preferred embodimentof the add switch 84 shown in FIG. 18, and FIG. 21B is a block diagramshowing a third preferred embodiment of the drop switch 86 shown in FIG.18. As shown in FIG. 21A, the add switch 84 includes two k×n opticalswitches 88(#1) and 88(#2) for a main system, and two k×n opticalswitches 90(#1) and 90(#2) for a standby system. The add switch 84further includes n 1×3 optical switches 122(#1) provided on the outputside of the optical switch 88(#1), n 1×3 optical switches 122(#2)provided on the output side of the optical switch 88(#2), n 3×1 opticalswitches 124(#1) provided on the output side of the optical switch90(#1), and n 3×1 optical switches 124(#2) provided on the output sideof the optical switch 90(#2), so as to switch each output of the opticalswitches 90(#1) and 90(#2) to each output of the optical switches 88(#1)and 88(#2). The operation of this preferred embodiment will beunderstood from the similarity to the operation of the preferredembodiment shown in FIG. 14A, so the description thereof will be omittedherein.

[0116] As shown in FIG. 21B, the drop switch 86 includes two n×k opticalswitches 96(#1) and 96(#2) for a main system, and two n×k opticalswitches 98(#1) and 98(#2) for a standby system. The drop switch 86further includes n 3×1 optical switches 128(#1) provided on the inputside of the optical switch 96(#1), n 3×1 optical switches 128(#2)provided on the input side of the optical switch 96(#2), n 1×3 opticalswitches 126(#1) provided on the input side of the optical switch98(#1), and n 1×3 optical switch 126(#2) provided on the input side ofthe optical switch 98(#2), so as to switch each input of the opticalswitches 96(#1) and 96(#2) to each input of the optical switches 98(#1)and 98(#2). The operation of this preferred embodiment will beunderstood from the similarity to the operation of the preferredembodiment shown in FIG. 14B, so the description thereof will be omittedherein.

[0117] In any case, the k×n optical switches 88(#1), 88(#2), 90(#1), and90(#2) control so that the wavelength conversion to a desired wavelengthin the ring is performed, and the n×k optical switches 96(#1), 96(#2),98(#1), and 98(#2) control so that an optical signal is output to adesired port.

[0118] Generally, the number of k of ports for adding/dropping issmaller than the number n of wavelengths of the WDM signal light, sothat (n−k) low-priority signals can be transmitted even in case offailure.

[0119]FIG. 22A is a block diagram showing a fourth preferred embodimentof the add switch 84 shown in FIG. 18, and FIG. 22B is a block diagramshowing a fourth preferred embodiment of the drop switch 86 shown inFIG. 18. In the preferred embodiment shown in FIG. 22A, 1×3 opticalcouplers 130(#1) and 130(#2) are used in place of the 1×3 opticalswitches 122(#1) and 122(#2) shown in FIG. 21A, respectively. In thepreferred embodiment shown in FIG. 22B, 1×3 optical couplers 132(#1) and132(#2) are used in place of the 1×3 optical switches 126(#1) and126(#2) shown in FIG. 21B, respectively. Also in these preferredembodiments, the switching in case of failure can be easily performed inthe optical add/drop device in a similar manner.

[0120] According to the present invention as described above, it ispossible to provide an optical add/drop device which can increase atransmission distance between nodes to thereby allow long-haultransmission. Further, it is also possible to provide a low-cost opticaladd/drop device which can eliminate the need for a tunable wavelengthconverter including a tunable light source at an interface between theoptical add/drop device and another network device. Further, it is alsopossible to improve the operation efficiency within an optical networkand to reduce the scale of hardware for switching.

[0121] The present invention is not limited to the details of the abovedescribed preferred embodiments. The scope of the invention is definedby the appended claims and all changes and modifications as fall withinthe equivalence of the scope of the claims are therefore to be embracedby the invention.

What is claimed is:
 1. An optical add/drop device comprising: an opticaldemultiplexer for separating WDM signal light into n (n is an integersatisfying 1<n) optical signals having different wavelengths, said WDMsignal light being obtained by wavelength division multiplexing said noptical signals; n first optical switches each having first and secondinput ports and first and second output ports, said n optical signalsoutput from said optical demultiplexer being supplied to said firstinput ports of said n first optical switches, respectively; a secondoptical switch having k (k is a natural number) input ports and n outputports, an optical signal to be added being supplied to at least one ofsaid k input ports of said second optical switch, said n output ports ofsaid second optical switch being connected to said second input ports ofsaid n first optical switches, respectively; n regenerators connected tosaid first output ports of said n first optical switches, respectively;an optical multiplexer for wavelength division multiplexing opticalsignals output from said n regenerators; and a third optical switchhaving n input ports and k output ports, said n input ports of saidthird optical switch being connected to said second output ports of saidn first optical switches, respectively, an optical signal to be droppedbeing output from at least one of said k output ports of said thirdoptical switch.
 2. An optical add/drop device according to claim 1,further comprising k wavelength converters connected to said k outputports of said third optical switch, respectively.
 3. An optical add/dropdevice according to claim 1, further comprising n wavelength convertersconnected between said n input ports of said third optical switch andsaid second output ports of said n first optical switches, respectively.4. An optical add/drop device according to claim 1, further comprising nwavelength converters connected between said optical demultiplexer andsaid first input ports of said n first optical switches, respectively.5. An optical add/drop device according to claim 1, wherein: said secondoptical switch further has r (r is a natural number) input ports; saidthird optical switch further has r output ports; and said opticaladd/drop device further comprises r links for connecting said r inputports of said second optical switch and said r output ports of saidthird optical switch.
 6. An optical add/drop device comprising: anoptical demultiplexer for separating WDM signal light into n (n is aninteger satisfying 1<n) optical signals having different wavelengths,said WDM signal light being obtained by wavelength division multiplexingsaid n optical signals; n optical switches each having first and secondinput ports and first and second output ports, said n optical signalsoutput from said optical demultiplexer being supplied to said firstinput ports of said n optical switches, respectively; a first electricalswitch having k (k is a natural number) input ports, s (s is a naturalnumber) input ports, and n output ports; k first opto/electricalconverters connected to said k input ports of said first electricalswitch, respectively; n first electro/optical converters for connectingsaid n output ports of said first electrical switch and said secondinput ports of said n optical switches, respectively; an opticalmultiplexer for wavelength division multiplexing optical signals outputfrom said first output ports of said n optical switches; a secondelectrical switch having n input ports, k output ports, and s outputports; n second opto/electrical converters for connecting said n inputports of said second electrical switch and said second output ports ofsaid n optical switches, respectively; k second electro/opticalconverters connected to said k output ports of said second electricalswitch, respectively; and s electrical links for connecting said s inputports of said first electrical switch and said s output ports of saidsecond electrical switch, respectively.
 7. An optical add/drop devicecomprising: an optical demultiplexer for separating WDM signal lightinto n (n is an integer satisfying 1<n) optical signals having differentwavelengths, said WDM signal light being obtained by wavelength divisionmultiplexing said n optical signals; n optical switches each havingfirst and second input ports and first and second output ports, said noptical signals output from said optical demultiplexer being supplied tosaid first input ports of said n optical switches, respectively; a firstelectrical switch having k (k is a natural number) input ports, s (s isa natural number) input ports, and n output ports; (k+s) firstopto/electrical converters connected to said (k+s) input ports of saidfirst electrical switch, respectively; n first electro/opticalconverters for connecting said n output ports of said first electricalswitch and said second input ports of said n optical switches,respectively; an optical multiplexer for wavelength divisionmultiplexing optical signals output from said first output ports of saidn optical switches; a second electrical switch having n input ports, koutput ports, and s output ports; n second opto/electrical convertersfor connecting said n input ports of said second electrical switch andsaid second output ports of said n optical switches, respectively; (k+s)second electro/optical converters connected to said (k+s) output portsof said second electrical switch, respectively; and s optical links forconnecting said first opto/electrical converters respectivelycorresponding to said s input ports of said first electrical switch andsaid second electro/optical converters respectively corresponding tosaid s output ports of said second electrical switch, respectively. 8.An optical add/drop device adapted to first and second optical paths,comprising: first and second optical add/drop units connected to saidfirst and second optical paths, respectively; an add switch forselectively adding an optical signal to any one of said first and secondoptical add/drop units; and a drop switch for selectively dropping anoptical signal from any one of said first and second optical add/dropunits; each of said first and second optical add/drop units comprising:an optical demultiplexer for separating WDM signal light into n (n is aninteger satisfying 1<n) optical signals having different wavelengths,said WDM signal light being obtained by wavelength division multiplexingsaid n optical signals; n optical switches each having first and secondinput ports and first and second output ports, said n optical signalsoutput from said optical demultiplexer being supplied to said firstinput ports of said n optical switches, respectively; n regeneratorsconnected to said first output ports of said n optical switches,respectively; and an optical multiplexer for wavelength divisionmultiplexing optical signals output from said n regenerators; said addswitch being connected to said second input ports of said n opticalswitches; said drop switch being connected to said second output portsof said n optical switches.